This invention relates to implants for total knee arthroplasty, and more particularly, to an improved stemmed tibial prosthetic implant.
Total knee arthroplasty involves providing new articulating surfaces for the tibia, femur, and patella. The most common technique for providing new articulating surfaces for the tibia involves resecting an upper portion of the tibia and then attaching a prosthetic implant to the tibia over the resected surface. The tibial implant includes a base adapted to abut the resected surface of the tibia and an articulating portion which includes the new articulating surfaces. The base of the implant has a shape that is adapted to generally approximate the shape of the resected surface of the tibia so that the implant generally aligns with .Iadd.and completely covers .Iaddend.the resected surface when the base is properly attached to the tibia.
The implant may also include a plurality of pegs extending from the lower or inferior surface of the base. The pegs are adapted to extend into the bone of the tibia when the implant is secured to the resected surface and provide enhanced torsional stability about the longitudinal axis of the tibia. In addition or alternatively to pegs, a stem may extend from the inferior surface of the base so as to extend a substantial distance into the tibia when the implant is attached over the resected surface. Tibial stems provide further torsional stability and axial strength by providing more surface area for contact with the bone. The stem also provides a "keel" effect to prevent the tibial implant from toggling or wobbling.
The tibia itself comprises an outer layer of hard cortical bone and a central canal of relatively soft cancellous bone. Much of the strength of the tibia is provided by the cortical bone. In a normally shaped tibia, the central canal is offset from the center of the tibial articulating surfaces or the center of the tibial plateau. The stems of prior tibial implants .Iadd.used for normally shaped tibias .Iaddend.were, however, positioned centrally to the implant base. Although the central location of the stem allowed a particular implant to be used for either the right or left knee, this stem position resulted in serious drawbacks. The primary drawback was that the centrally located stem was substantially offset from the center of the tibial canal itself when the base of the implant was aligned with the resected tibial surface. In fact, stems located centrally to the base occasionally contacted the posterior .Iadd.and/or lateral .Iaddend.cortical bone of the tibia. This interference with the cortical bone sometimes prevented the base of the implant from seating flush against the resected surface of the tibia, thereby inhibiting adequate initial stability and fixation which is essential for successful knee arthroplasty.